1. Field of the Invention
The present invention relates to an ink jet printer, and more particularly, to a method and related apparatus for detecting short-circuit and open-circuit problems of inkjet units of the inkjet printer.
2. Description of the Prior Art
Having advantages of low-cost and excellent printing performance, ink jet printers have become one of the most popular electrical output devices.
Please refer to FIG. 1, which is a schematic diagram of a circuit of a prior art printer 10 (U.S. Pat. No. 5,736,997). The printer 10 comprises a plurality of ink jet units A11 to A13, A21 to A23, and A31 to A33 disposed in matrix. The printer 10 further comprises a controller 12 for controlling the functionality of the printer 10, a power circuit 14 for providing energy to the ink jet units, an address circuit 16A for selectively controlling the ink jet units, a driving circuit 16B for selectively driving the ink jet units, and a detecting circuit 18. As each ink jet unit has the same structure, the ink jet unit A13 is described as an example. The ink jet unit A13 comprises a field effect transistor T, a heating element D, and a corresponding nozzle K. A gate electrode Tc of the transistor T serves as a control end of the ink jet unit A13. A source electrode of the ink jet unit A13 and a drain electrode of the ink jet unit A13 are respectively connected to ground and to one end of the heating element D. The other end of the heating element D is connected to a node Ti and serves as an input end of the ink jet unit A13. The heating element D is usually a resistor and is installed inside an ink container (not shown) of the ink jet unit A13 for transforming electrical energy into heat energy to heat the ink stored in the ink container when currents are flowing through the heating element D. When the temperature of the ink exceeds a threshold, the ink will spray via the corresponding nozzle K.
The address circuit 16A comprises three address lines Aa1, Aa2, and Aa3 corresponding to the three-row disposition of the plurality of ink jet units A11 to A13, A21 to A23, and A31 to A33. The address line Aa1 is connected to the control ends of the ink jet units A11, A12, and A13. The address line Aa2 is connected to the control ends of the ink jet units A21, A22, and A23. The address line Aa3 is connected to the control ends of the ink jet units A31, A32, and A33. The driving circuit 16B also comprises three driving lines Pa1, Pa2, and Pa3 corresponding to the three-column disposition of the plurality of ink jet units A11 to A13, A21 to A23, and A31 to A33. The driving line Pa1 is connected to the input ends of the ink jet units A11, A21, and A31. The driving line Pa2 is connected to the control ends of the ink jet units A12, A22, and A32. The driving line Pa3 is connected to the control ends of the ink jet units A13, A23, and A33. The controller 12 of the printer 10 determines the functionality of the ink jet units by controlling the address circuit 16A and by controlling the driving circuit 16B. For example, when the controller 12 determines that the ink jet unit A13 sprays ink, the address circuit 16A raises the voltage level of the address Aa1 by using the energy provided by the power circuit 14. The high level voltage of the address line Aa1 actuates the transistor T. At the same time, the driving circuit 16B raises the voltage of the driving line Pa3 also by using the energy provided by the power circuit 14. Then driving currents Id generated by the power circuit 14 flow through the driving line Pa3, to node Ti, and finally into the ink jet unit A13. The heating element D of the ink jet unit A13 transforms the electrical energy of the driving currents Id into heat energy, which will heat the ink stored in the ink jet unit A13 and will make the nozzle K of the ink jet unit A13 spray ink. On the contrary, when the controller 12 determines that the ink jet unit A12 needs not spray ink, the controller 12 will controls the address circuit 16A to keep the voltage of the ink jet unit A12 at low level. Thus, when the voltage of the address line Aa1 is at high level, the driving line Pa2 will not convey any currents to the ink jet unit A12. When the controller 12 also determines that the ink jet unit A23 needs not spray ink, the controller 12 will control the address circuit 16B to keep the voltage of the address line Aa2 at low level. The low level voltage of the address line Aa2 is not capable of actuating the transistor T of the ink jet unit Aa2, so the ink jet unit A23 still does not spray any ink even when the driving line Pa3 has been kept at high level.
Using the above-mentioned controlling process, the printer 10 is capable of controlling individual ink jet unit to accurately spray ink according to an image. However, the printer 10 usually has some circuit problems. For example, the inkjet unit A33 of the printer 10 has an open-circuit (OC) problem. No matter what voltage level the address line Aa3 is at, the driving-energy provided by the power circuit 14 will not flow through the driving line Pa3 into the ink jet unit A33. That is, the printer 10 is not capable of effectively controlling the functionality of the ink jet unit A33. As another example, the ink jet unit A32 of the printer 10 has a short-circuit (SC) problem (possibly due to a breakdown of the transistor T or to a malfunction of the heating element D). No matter what voltage level the address line Aa3 is at, whenever the driving circuit 16B raises the voltage of the driving line Pa2, because the driving line Pa2 is shorted to the ground, the currents flowing through the driving line Pa2 become extremely high. The extremely high current may damage the control logic circuit of the driving circuit 16B or may further damage the power circuit 14. Thus far the printer 10 is useless.
To detect the above short-circuit problem, the prior art printer 10 relies on a detecting circuit 18, as shown in FIG. 1. The detecting circuit 18 of the printer 10 comprises three diodes D1 to D3 respectively connected to the three driving lines Pa1 to Pa3. The detecting circuit 18 further comprises a comparator Vc1 for generating a detecting signal 18S. The comparator Vc1 has a positive end and a negative end. The negative end of the comparator Vc1 is connected to a resistor R1 and to the anodes of the three diodes D1 to D3. The positive end of the comparator Vc1 is connected to a voltage divider composed of a voltage source Vcc and two resistors R2, R3 by a contact point of the two resistors R2, R3. The functionalities of the detecting circuit 18 are described as follows. If the printer 10 functions normally (that is, no short-circuit problem), the three diodes D1 to D3 will be reverse biased and no currents will flow through the resistor R1. So the voltage of the negative end of the comparator Vc1 equals Vcc. In the meantime, the voltage of the positive end of the comparator Vc1 is always lower than Vcc due to the voltage dividing effect of the two resistors R2, R3. Therefore, the comparator Vc1 is capable of determining whether the printer 10 has any short-circuit problems by comparing the voltage of the positive end with the voltage of the negative end. If all the ink jet units of the printer 10 function normally, the detecting circuit 18 generates a corresponding functioning-normally detecting signal 18S. On the contrary, if the ink jet unit A32 has a short-circuit problem, as described previously, the voltage of the driving line Pa2 is decreasing, so the diode D2 actuates and currents flow through the resistor R1. Thus the voltage of the negative end of the comparator Vc1 is decreased. If the voltage of the negative end is lower than that of the positive end, the comparator Vc1 then determines that the printer 10 has a short-circuit problem.
Although the detecting circuit 18 of the printer 10 is capable of detecting a short-circuit problem, the previously-mentioned extremely high currents could have possibly already destroyed the address circuit 16A, the driving circuit 16B, and the power circuit 14 before the detecting circuit generates the detecting signal 18S (has detected the short-circuit problem). Furthermore, when the driving circuit 16B is raising the voltage of the driving line Pa3, the open-circuit effect occurring in the ink jet unit A33 will not affect the voltage of the driving line Pa3, so the detecting circuit 18 is not capable of detecting an open-circuit problem of any ink jet unit of the printer 10.